Magnetic tunneling junction (MTJ) devices are made of two ferromagnetic layers separated by a thin, non-magnetic insulating layer. MTJ devices are used in magnetoresistive (MR) heads and in magnetic random access memory (MRAM) devices.
The two ferromagnetic layers of a MTJ are selected to have different responses to magnetic fields, so that the relative orientation of their magnetic moments can be varied with an external magnetic field. One ferromagnetic layer of a MTJ device has a fixed magnetic orientation, while the other ferromagnetic layer is free to change its magnetic orientation. These two ferromagnetic layers are usually called the fixed ferromagnetic layer and the free ferromagnetic layer.
The insulating layer that separates the fixed and free ferromagnetic layers is usually called the tunneling barrier layer. The tunneling barrier layer derives its name from the fact that it is sufficiently thin that electrons are able to quantum-mechanically tunnel through it. When a voltage is applied across the pair of ferromagnetic layers, electrons travel between the two magnetic layers by tunneling through the tunneling layer.
The current through a MTJ device depends on the relative magnetic orientation of the fixed ferromagnetic layer and the free ferromagnetic layer. Resistance to the current is a maximum when the magnetic orientations of the two ferromagnetic layers are anti-parallel. Resistance is a minimum when the magnetic orientations of the two ferromagnetic layers are parallel. The difference between the maximum and minimum resistance divided by the resistance of the device (ΔR/R) is commonly referred to as the magnetoresistance (MR) ratio. A large MR ratio is desirable to maximize the signal to noise ratio (SNR).
Another important parameter of a MTJ device is the resistance area (RA) product. Because the two ferromagnetic layers of a MTJ device are excellent conductors and can be considered to have no resistance, the resistance of the insulating tunnel barrier layer of a device determines the total resistance of a MTJ device. Low resistance is desirable in a MTJ device. However, the resistance of a MTJ device increases as the area of the device decreases. Thus, as efforts are made to decrease the size of MTJ devices, the resistance of the devices increases.
Because resistance goes up as area goes down in a MTJ device, these two parameters are inherently in conflict. Therefore, the product of resistance multiplied by area, the RA product, is useful as a parameter for MTJ devices. A small RA product is desirable.
The tunneling barrier layer is the most important layer in a MTJ device, because performance of the device is highly dependent on the tunneling barrier material. The material most commonly used for the tunneling barrier layer of an MTJ device is aluminum oxide (Al2O3). However, RA products increase exponentially with the thickness of an Al2O3 tunneling barrier layer. Since low RA products are necessary for applications such as recording heads, and creating very thin layers is problematic, Al2O3 has not been successful as a tunneling barrier layer in low RA applications.
Recently, gallium oxide (Ga2O3) has emerged as an alternative to aluminum oxide for use as the tunneling barrier layer of a MTJ device. For example, U.S. Pat. No. 6,359,289 discloses use of a gallium oxide tunneling barrier layer. MTJ devices made with a gallium oxide tunneling barrier layer disclosed in the '289 patent have lower RA products, while maintaining relatively high MR ratios. However, for MTJ devices to be useful as read heads, even lower RA products are needed.
The most common and successful method of forming a tunneling barrier layer consists of depositing a metal layer and then using either natural oxidation or plasma oxidation to create the metal oxide. For example, the gallium oxide tunneling barrier layer disclosed in the '289 patent is formed by sputter deposition of gallium followed by plasma oxidation to create gallium oxide. MTJ devices with gallium oxide tunneling barrier layers disclosed in the '289 patent show maximum MR ratios of 22% at room temperature, with RA products of 800,000Ω·μm2. A MTJ device with such a large RA product would not be useful as a recording head in a magnetic storage device.
Therefore, in order to meet the requirements for use of MTJ devices in recording heads, it is desirable to develop MTJ devices with gallium oxide tunneling barrier layers that have lower RA products, while maintaining high MR ratios. Also, in order to control stack uniformity, it is desirable to form a tunneling barrier layer without oxidation or nitridation. It is also desirable to have a thick barrier, without sacrificing RA products or MR ratios.